Carbon fibers are widely used as reinforcement for advanced polymeric matrix composites in many high-technology applications because of their high specific stiffness, strength, and excellent electrical and thermal properties. Carbon fibers have many forms, depending on the synthesis methods used. Conventional carbon fibers such as those obtained by pyrolyzing polyacrylonitrile (PAN) resins or other precursors are typically several microns in width and have high aspect ratios. The discovery of carbon nanotubes, single-walled or multi-walled carbon nanotubes (SWNTs or MWNTs), has led to the fabrication of various kinds of nanocomposites. Graphitic carbon nanofibers (GCNFs) are also attractive additives for fabricating carbon fiber/polymer composite materials of enhanced strength and electrical conductivity due to their unusual atomic structure.
The unique properties of graphitic carbon nanofibers have generated intense interest in the application of these new carbon materials in a number of applications, including polymer reinforcement. The preparation and characterization of GCNFs has been well studied and such fibers are now available commercially. Three types of GCNFs, “platelet” (perpendicular), “ribbon” (parallel) and “herringbone” structures, are grown via catalytic decomposition of hydrocarbons or carbon monoxide over metal nanoparticle growth catalysts. The width of the nanofibers produced is controlled by the size of the catalyst particle responsible for their growth and can vary between 5 nm and hundreds of nanometers, with lengths ranging from 5-10 microns. Several carbon nanofiber/organic polymer nanocomposites are known. For example, 5 wt % carbon nanofiber/polypropylene composites exhibit tensile strength and modulus enhancements of 20% and 50%, respectively. Carbon nanofiber reinforced poly(ether ketone) composites are also known, and evaluation of the mechanical properties of such composites reveals a linear increase in tensile stiffness and strength, with nanofiber loading up to 15 wt %. Those studies show that efficient wetting and high dispersion of carbon nanofibers within a polymer matrix continues to be problematic.
Chemical modification of carbon fiber surfaces can be used to enhance attractive interactions at the fiber/polymer interface. Surface-derivatization of conventional carbon fibers with hydroxyl, ketone, carboxyl or amino groups leads to significant improvement in fiber wettability and fiber-matrix adhesion, resulting in increased interlaminar shear strength and flexural strength. With nanoscale carbon fibers now available, it is possible to control fiber/matrix interactions at nearly the atomic level.
Obtaining uniform dispersions of nanofiber reinforcement within a polymer-matrix is another critical issue in nanocomposite processing. The degree to which nanofiber additives can be homogeneously dispersed in a matrix strongly influences the degree of property enhancement of the resulting composite. Nanoscale particles tend to aggregate due to their high surface areas, high aspect ratios, and a thermodynamic driving force to maximize fiber-fiber electrostatic and van der Waals interactive forces. Nanofiber agglomeration inhibits their uniform dispersion in composite materials and prevents efficient transfer of nanofiber properties to the composite matrix. Greatly enhanced performance of nanocomposite materials reinforced with nanotube or nanofiber additives has not been fully achieved because of difficulties in achieving efficient dispersion and wetting of the nanoscale component within the matrix material, even when using surface-functionalized additives.
While GCNFs have atomic structures amenable to high-density surface-derivatization, full realization of this advantage critically depends on achieving complete dispersion of these nanofibers in the matrix. To obtain high-quality nanocomposites with excellent properties, a technique of uniformly dispersing functionalized GCNFs during processing is required. Known methods for dispersing nanoparticles aggregates include; (1) mechanical agitation such as stirring, (2) use of dispersing agents, and, (3) ultrasonic vibration. Ultrasonication has also been used to disperse sub-micrometer powders that are difficult to disperse by other methods.
Fiber-matrix interfacial adhesion can play an important role in determining the mechanical properties of carbon fiber/polymer composites. Stronger interfacial bonding generally imparts better mechanical properties. Surface modification of carbon fibers can improve bonding between the fiber surface and polymer resin components.
For conventional carbon/graphite fibers, as mentioned above, it is known that surface derivatization, as well variety of surface coating and modification techniques, can be used to enhance interfacial bonding between carbon fiber additives and polymer matrices. Factors that can be significant in promoting adhesion in such materials include, for example: 1) removal of any weak boundary layers; 2) introduction of variable surface topography to enhance mechanical interlocking; 3) improved fiber wettability; and, 4) creation or addition of chemical functional groups on fiber surfaces. Surface-derivatization of conventional carbon fibers with hydroxyl, carboxyl, or amino groups can result in significant improvement in fiber wettability and fiber-matrix adhesion resulting in greater interlaminar shear and flexural strength. Surface derivatization has not previously been reported for carbon nanofibers having reactive surface carbons.
It is an object of the present invention to provide surface-functionalized and surface-derivatized graphite carbon nanofibers (GCNF).
It is a further object of the present invention to provide methods for producing surface-functionalized and surface-derivatized GCNF.
It is still a further object of the invention to provide GCNF-reinforced polymer/resin composites utilizing the above derivatized GCNF.
It is another object of the invention to provide a method of forming the above composites.
Still another object of the invention is to provide articles of manufacture comprising either the above surface-functionalized GCNF, surface-derivatized GCNF or the composites formed therefrom.